This invention relates to methods for coating a substrate with a film of silica. More particularly, this invention relates to such a process which increases the uniformity of the film on the substrate which property of uniformity has the effect of increasing substantially the active surface area of the silica. More particularly, this invention relates to such a process for use in, for example, coating blood sample collection tubes such as serum separation tubes to increase the clot activation properties thereof once a blood sample is introduced into the tube.
The length of time required for blood collected in a blood collection assembly to clot after it is introduced into the assembly is dependent upon a number of interrelated factors. One of the factors which increases the rate of clot formation is exposure of the blood to "siliceous" materials such as glass, silica, kaolin, bentonite or diatomaceous earth. Therefore, it is important that the exposure of the blood sample to the presence of a silica in a sample containing tube be as uniform and rapid as possible.
Representative prior art patents which teach among other things, silica coatings in blood collection tubes, such as serum separation tubes, include U.S. Pat. Nos. 4,153,739 and 4,257,886. The '739 patent teaches a clot activating film 24 with the film being formed of 1% by weight polyvinyl pyrrolidone and 1% by weight silica added to isopropanol. In substitution for polyvinyl pyrrolidone, polyethylene oxide may be used. The '886 patent teaches, on the other hand, a water-soluble clot activating coating 36 wherein the coating is comprised of an admixture of polyvinyl pyrrolidone or polyethylene oxide with clot activating particles such as silica in a solvent such as isopropanol. In both cases, the source of the silica is fine silica particles.
While the methods taught in the two patents have proved satisfactory in the sense that they produce coatings on substrates such as serum separation tubes which coatings contain silica for clot activation, the films have not proved entirely satisfactory in the sense that they are not as uniform as desired. Because of this, there is less surface active area for bringing about the desired clot activation. It has been found that one way to overcome this problem in order to achieve the best coatings, it is necessary to include Freon in combination with the isopropyl alcohol in order to secure a proper uniform coating. Freon also lowers the flash point of isopropyl alcohol during the drying cycle. However, Freon is expensive and creates corrosion problems in processing equipment for obtaining the coatings.